violent engine start and backfiring
 

I own an 05 porsche carerra 4s, automatic transmission. When I go to start it after one to two days of not driving - the engine shakes violently and it backfires really loud. The engine stops shaking and backfiring after about twenty to thirty seconds and then everything seems to be o.k. This has happened five times in three months. It drives beautifully - no problems on the road. I've had the car for three months and three different porsche service centers have not been able to duplicate the problem and they don't know what is wrong. Even when I brought it into one dealership with the warning light still on that said "bring to repair shop" - they hooked up their computer to the car and they still couldn't tell me what was wrong. I am in southern california - no cold weather and I park the car in a covered parking structure. Please - Does anybody know what might cause this problem. - Thank You, Dennis Liss